The present invention relates generally to the aging of proteins resulting from reaction of glucose, and particularly to the nonenzymatic glycosylation of proteins and subsequent reactions leading to advanced glycosylation end products, and to methods and agents for their inhibition.
The reaction between glucose and proteins has been known for some time. Its earliest manifestation was in the appearance of brown pigments during the cooking of food, which was identified by Maillard in 1912, who observed that glucose or other reducing sugars react with amino acids to form adducts that undergo a series of dehydrations and rearrangements to form stable brown pigments, Maillard, L.D. C.R. Acad. Sci., 154, pp. 66-68 (1912).
In the years that followed the initial discovery by Maillard, food chemists studied the hypothesized reaction in detail and determined that stored and heat treated foods undergo nonenzymatic browning as a result of the reaction between glucose and the polypeptide chain, and that the proteins are resultingly crosslinked and correspondingly exhibit decreased bioavailability. At this point, it was determined that the pigments responsible for the development of the brown color that develops as a result of protein glycosylation possessed characteristic spectra and fluorescent properties; however, the chemical structure of the pigments had not been specifically elucidated.
The reaction between reducing sugars and food proteins discussed above was found in recent years to have its parallel in vivo. Thus, the nonenzymatic reaction between glucose and the free amino groups on proteins to form a stable amino, 1-deoxy ketosyl adduct, known as the Amadori product, has been shown to occur with hemoglobin, wherein a rearrangement of the amino terminal of the .beta.-chain of hemoglobin by reaction with glucose, forms the adduct known as hemoglobin A.sub.1c. The reaction has also been found to occur with a variety of other body proteins, such as lens crystallins, collagen and nerve proteins. See Bunn, H.F., Haney, D.N., Gabbay, K.H. and Gallop, P.H., Biochem. Biophys. Res. Comm., 67, pp. 103-109 (1975); Koenig, R.J., Blobstein, S.H. and Cerami, A., J. Biol. Chem., 2.52, pp. 2992-2997 (1975); Monnier, V.M. and Cerami, A. in Maillard Reaction in Food and Nutrition, ed. Waller, G.A., American Chemical Society, 215, pp. 431-448 (1983); and Monnier, V.M. and Cerami, A., Clinics in Endocrinology and Metabolism, 11, pp. 431-452 (1982).
Moreover, brown pigments with spectral and fluorescent properties similar to those of late-stage Maillard products have also been observed in vivo in association with several long-lived proteins, such as lens proteins and collagen from aged individuals. An age-related linear increase in pigment was observed in human dura collagen between the ages of 20 to 90 years. See Monnier, V.M. and Cerami, A., Science, 211, pp. 491-493 (1981); Monnier, V.M. and Cerami, A., Biochem. Biophys. Acta, 760, pp. 97-103 (1983); and Monnier, V.M., Kohn, R.R. and Cerami, A., "Accelerated Age-Related Browning of Human Collagen in Diabetes Mellitus", Proc. Natl. Acad. Sci., 81, pp. 583-587 (1984). Interestingly, the aging of collagen can be mimicked in vitro by the crosslinking induced by glucose; and the capture of other proteins and the formation of adducts by collagen, also noted, is theorized to occur by a crosslinking reaction, and is believed to account for the observed accumulation of albumin and antibodies in kidney basement membrane. See Brownlee, M., Pongor, S. and Cerami, A., J. Exp. Med., 158, pp. 1739-1744 (1983); and Kohn, R. R., Cerami, A. and Monnier, V.M., Diabetes, 33(1), pp. 57-59 (1984).
Recently, the role of other naturally-occurring reducing sugars, including fructose, in nonenzymatic cross-linking has been discussed. Specifically, Suarez et al. "Administration of an Aldose Reductase Inhibitor Induces a Decrease of Collagen Fluorescence in Diabetic Rats", J. Clin. Invest, 82, pp. 624-627 (1988) have shown that fructose levels are elevated in diabetes as a result of the elevated glucose being channeled through the polyol pathway, first to sorbitol then to fructose. These investigators also showed that the ability of fructose to cause nonenzymatic crosslinking as measured by collagen fluorescence, is 10 times greater than that of glucose. Because the methods and agents of the present invention block nonenzymatic crosslinking mediated by any of the reactive sugars, they are expected to prevent fructose-mediated crosslinking as well. Cross-linking caused by other reactive sugars present in vivo or in foodstuffs, including ribose and galactose, would also be prevented by the methods and compositions of the present invention.
In parent application Ser. No. 590,820 (now U.S. Pat. No. 4,665,192) and in Pongor, S.M., et al., supra, both incorporated herein by reference, a fluorescent chromophore was isolated and identified which was found to be present in certain browned polypeptides such as bovine serum albumin and poly-L-lysine, and was assigned the structure 2-(2-furoyl)-4(5)-2(furanyl)-1H-imidazole. The compound was found to exist in a tautomeric state and has incorporated in its structure two peptide-derived amine nitrogens. The incorporation of these amine nitrogens and two glucose residues in the compound suggested that its peptide-bound precursor may be implicated in the in vivo crosslinking of proteins by glucose, which is observed in the late stage of the Maillard process. [See Chang, J.C.F., Ulrich, P.C., Bucala, R. and Cerami, A., J. Biol. Chem., 260, pp. 7970-7974 (1985)]. This chromophore made possible the identification of the advanced glycosylation end products and assisted additional investigations seeking to clarify the protein aging process and to identify the specific chemistry involved in order to develop methods and agents for its inhibition. It is to this purpose that the present Application is directed.
More recently, other advanced glycosylation products have been identified, such as Farmar et al., U.S. Ser. No. 097,856, filed Sep. 17, 1987; pyrraline (Hayase et al., "Aging of Proteins: Immunological Detection of a Glucose-derived Pyrrole Formed during Maillard Reaction in Vivo", J. Biol. Chem., 263, pp. 3758-3764 (1989)), pentosidine (Sell, D. and Monnier V. "Structure Elucidation of a Senescence Cross-link from Human Extracellular Matrix" J. Biol. Chem., 264, pp. 21597-21602 (1989)). Formation of these advanced glycosylation products would be inhibited by the methods and agents of the present invention. The present invention is not restricted to any of these advanced glycosylation endproducts specifically, but to the general process of their formation as a result of the reaction of proteins with sugars.